1. A well-known example of this method is to introduce, in addition to positions of points, the components of their velocities when describing a state of a certain mechanical system.
2. I. ‘Théorie de la spéculation’, Ann. École Norm. Supér.
17 (1900), 21; II. ‘Les probabilités à plusieurs variables’, Ann. École Norm. Super.
27 (1910), 339; III. Calcul des probabilités, Paris, 1912.
3. Concerning these notions, as well as additive sets of systems, etc., see, for example, M. Fréchet,’ sur l’intégrale d’une fonctionnelle étendu à un ensemble abstrait’, Bull. Soc. Math. France
43 (1915), 248.
4. See the first of the papers cited in footnote 2.
5. C. R. Acad. Sci. Paris186 (1928), 59; 189; 275.